Our goals are: 1) determine the density and distribution of Ig, Fc receptor and AgB on aged and young B and T lymphocytes and macrophages; 2) compare the kinetics of capping and endocytosis and their inhibition on the above 3 effector elements; 3) compare the organization of microfilaments and microtubules in aged and young effector cells, without and with capping; 4) relate the effects of capping, endocytosis, and structural changes of microfilaments and microtubules to functional activities of the above 3 effector elements. "Pure" populations of B and T lymphocytes and macrophages are reacted with fluorescein or isotope-labeled antibodies specific for Ig, Fc receptor (Ag-Ab complexes) and AgB for examination in fluorescence, scanning and electron microscopes. Microfilaments and microtubules are visualized by use of fluorescein-labeled antibody specific for actin or myosin and tubulin, and by ultrastructural analysis. Enriched cell populations are derived from control donor rats and from rats primed with tumor- and viral-associated antigens. Functional studies include antibody formation by B cells; cell-mediated cytotoxicity to tumor target cells, mixed lymphocyte reactions, and response to antigen by T cells; and phagocytosis, release of beta-glucuronidase, and antigen digestion by macrophages. Functional assays are carried out on "pure" populations of cells in which capping and endocytosis have been induced and in which microfilaments and microtubules have been disrupted by agents such as cytochalasins, colchicine, and Ca2 ion.